Between January, 1982 and April, 1983, 92 adult patients with acute leukaemia were investigated in our department. According to classical criteria (cytology and optical cytochemistry), 34 were classified as "non-myeloid". These were further tested with a panel of monoclonal antibodies against cell membrane, including ALB1-2 (anti-CALLA), ALB6 (anti-p24), ALB7-8-9 (BA1), OKT and HLA DR; they were also tested for E rosettes, Slg and terminal transferase (TDT). When all these markers but HLA DR were negative, patients were investigated for ultrastructural peroxidases which were found to be present in 2 cases. Among all non-myeloid leukaemias, 18 (56%) were CALLA-positive and TDT-positive acute lymphoblastic leukaemias (ALL), 2 (6%) were ALL T, 7 (22%) were CALLA-negative and TDT-positive ALL and 5 (16%) were acute leukaemias null for our markers, a phenomenon the significance of which is discussed. Patients with the CALLA-negative TDT-positive phenotype were peculiar with regard to age (mean: 50 years), female predominance, L2 cytological pattern according to the FAB classification, good prognosis (complete remission in 100% of the cases) and median survival (p less than 0,03).